Scrape-off layer measurements in DIII-D

Although the research on divertors and scrape-off layers (SOLs) has been not as focused on as the recent success of the Korean fusion program, a few linear plasma devices have been developed for simulating divertor and SOL plasmas: (1) diversified plasma simulator (DiPS), a versatile linear machine, has been developed for simulations of divertor and space plasmas with various electric probes, such as single, triple, and Mach Probes and gridded energy analyzer. DiPS consists of two major parts: a divertor plasma simulator with a \(\hbox {LaB}_6\) DC plasma source and a space plasma simulator with a helicon RF plasma source, (2) divertor plasma simulator-1 (DiPS-1) is a part of DiPS with only a \(\hbox {LaB}_6\) cathode, where a low-power laser-induced fluorescence (LIF) is added and more electric probe diagnostics are augmented; it is dedicated only for fusion edge and divertor plasmas, (3) Divertor Plasma Simulator-2 (DiPS-2) has been modified from the DiPS-1 by adding a magnetic nozzle with a limiter structure and by removing the helicon source and space chamber. DiPS-2 is a linear plasma device with a 4-inch \(\hbox {LaB}_6\) cathode, the same as DiPS-1, and it is focused on the development of various diagnostics, such as those used for LIF and laser Thomson scattering (LTS) along with various electric probes, on the divertor and scrape-off plasmas and on the plasma-material interaction (PMI) research, such as that of tungsten and graphite as plasma-facing components (PFCs), (4) A Multi-Purpose Plasma (\(\hbox {MP}^2\)) device is a renovation of the Hanbit mirror device [Kwon et al., Nucl. Fusion 43, 686 (2003)] with the installation of two plasma sources: \(\hbox {LaB}_6\) (DC) and helicon (RF) plasma sources. A honeycomb-like large-area \(\hbox {LaB}_6\) (HLA-\(\hbox {LaB}_6\)) cathode has been developed for the divertor plasma simulation to improve the resistance against the thermal shock fragility for large (8-inch) and high density plasma generation, (5) DiPS-2 has been augmented by adding another cylindrical device, called the Dust interaction with Surfaces Chamber (DiSC) for the generation and diagnostics of dusts. This combined system (DiPS-2+DiSC) has added two more diagnostics: Laser Photo-Detachment (LPD) for dust density and laser Mie Scattering (LMS) for dust size. Moreover, dusts or negative ions have been analyzed by using electric probes and capacitive diagram gauges in Transport and Removal of Dusts (TReD) device.

The mechanism underlying atherosclerotic ischemic events within the middle cerebral artery (MCA) is unclear. High structural stress induced by blood pressure might be a potential aetiology as plaque rupture occurs when such mechanical loading exceeds its material strength. To perform reliable analyses quantifying the mechanical loading within a plaque, the local blood pressure is needed. However, data on MCA blood pressure is currently lacking. In this study, the arterial pressure proximal to the stenotic site in the MCA was measured in 15 patients scheduled for intervention. The relationships between these local measurements and pre-intervention and intra-intervention non-invasive arm measurements were assessed. The impact of luminal stenosis on the local blood pressure was quantified. Compared with the pre-intervention arm measurement, the intra-intervention arm pressure decreased significantly by 23.9 ± 11.8 and 9.3 ± 14.7 % at diastole and systole, respectively. The pressure proximal to the stenosis was much lower than the pre-intervention arm measurement (diastole: 65.3 ± 15.7 vs 82.0 ± 9.7, p < 0.01; systole: 81.1 ± 15.9 vs 133.9 ± 18.7, p < 0.01; unit: mmHg). The systolic pressure in the MCA in patients with stenosis <70 % (n = 6) was significantly higher than the value in patients with stenosis ≥70 % (n = 9) (92.0 ± 7.3 vs 73.9 ± 16.1, p = 0.02; unit: mmHg), as was pulse pressure (22.8 ± 6.4 vs 11.1 ± 8.3, p = 0.01; unit: mmHg). However, diastolic pressure remained unaffected (69.2 ± 9.3 vs 62.8 ± 19.0, p = 0.58; unit: mmHg). In conclusion, the obtained results are helpful in understanding the local hemodynamic environment modulated by the presence of atherosclerosis. The local pressure measurements can be used for computational analysis to quantify the critical mechanical condition within an MCA lesion.

Two-dimensional maps of the Experimental Advanced Superconducting Tokamak (EAST) scrape-off layer (SOL) plasma conditions for ohmic, L-mode and H-mode discharges are reconstructed using an onion skin model (OSM) coupled in DIVIMP together with the Monte Carlo neutral transport code, EIRENE. The boundary conditions for OSM calculation are taken from the measurements of the Langmuir probe built into the divertor targets. The OSM-calculated values of the outboard mid-plane electron density, ne, and temperature, Te, are compared with the mid-plane measurements of ne and Te from a fast reciprocating probe. Some other characteristics of these SOL plasmas are also derived from the OSM solution, reflecting that the upstream plasma conditions are governed by the SOL collisionality to a large degree. Values of at the low-field side and the high-field side mid-plane are derived separately as a function of the distance to the separatrix for ohmic, L- and H-mode discharges, showing that increases with the distance to the separatrix at both sides and that the values of at the low-field side tends to be higher than that at the high-field side. is found to be larger than by a factor of 2–3 for all the discharges considered here. In addition, before the use of the OSM method of extracting and for EAST discharges, the reliability of this method is assessed by taking SOLPS-generated target n, T profiles as boundary conditions and by comparing the OSM-extracted cross-field transport coefficients with those input in the SOLPS modelling.

Measurements of toroidal flow velocities in the midplane of the inner and outer scrape-off layers (SOL) of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas are made using a high-resolution spectrograph. Doppler shifts of the line emissions from either He+ at 468.6 nm or neutral deuterium at 656.1 nm from two local gas puffs are observed along radially separated, toroidally viewing chords to obtain the radial profiles of the toroidal ion and neutral flow velocities and of the temperatures in the midplane of the inner and outer SOL’s. In these experiments, the ion vertical drift due to the toroidal magnetic field gradient (ion B×∇B drift) is directed towards the lower X-point, and an outer SOL plasma flow with a velocity in the range of 5–10 km/s in the direction of the plasma current is observed at the midplane far above the divertor plates. Plasma flow in a similar direction with similar velocities is observed in the inner SOL midplane. This observation suggests a toroidal rotation of the edge plasma in a co-current direction, consistent with an Er×Bθ motion that would arise from a radially outward electric field. Interestingly, the neutral atoms in these views are observed to flow in the opposite directions in the inner and outer SOL’s, i.e., the neutrals flow in the plasma current direction in the inner SOL and in a direction opposite to the plasma current in the outer SOL. Also, the neutrals move much slower (velocities ∼2–4 km/s) in the outer SOL compared to the inner SOL (velocities ∼4–10 km/s). Temperatures determined from the Doppler broadening of the spectral lines are found to be in the range of ∼15–25 eV for the He+ ions in the inner and outer SOL’s. Much lower temperatures of ⩽2–5 eV are indicated for the deuterium atoms in both SOL’s.

DIII-D edge physics data base

To explore the influence of drifts on ion flows in the DIII-D divertor and Scrape-Off-Layer (SOL), a calibrated Coherence Imaging System (CIS) views the lower divertor. A new tuneable calibration laser and precision wavemeter provide the rest wavelength for CIII 465 nm plasma emissions resulting in ≤ 1 km/s uncertainty in ion velocities that are in the range of 30 km/s in the divertor plasma. The 2-D line-integrated velocity images have been tomographically inverted to obtain maps of ion velocity. Both He + and C 2 + ion velocity maps were measured in both directions of the toroidal magnetic field BT, and C 2 + images were obtained in attached and detached L- and H-mode plasmas. The measured C 2 + velocities are in the range of 20–30 km/s and are compared with UEDGE modeling with full drifts. While He II as a main ion is expected to be entrained in the plasma flow, the C 2 + impurity flow involves the competition between the frictional and ion temperature gradient forces. Both the CIS data and UEDGE modeling agree that the C 2 + flow is towards the divertor plates, but flips with the direction of BT. In general, the experimental C 2 + emission and flow profiles have a wider spatial extent than the UEDGE modeling.

The JET scrape-off layer (SOL) has been characterized with a reciprocating probe in inner wall (IW), and outer wall (OW), limited plasmas. Experiments revealed that SOL profiles are substantially broader (by a factor of ∼5–7.5 in the power e-folding length) for IW limited than in OW limited plasmas. Results are consistent with the larger radial turbulent transport found for IW limited plasmas. Major differences are observed between IW and OW limited plasmas on the density and electron temperature e-folding lengths, parallel flow, radial turbulent transport as well as on the temporal and spatial characteristics of the fluctuations. Experimental findings on JET suggest that the differences in the SOL characteristics for both configurations are due to a combination of a poloidal asymmetry in radial transport with a reduced cross-field transport across the last closed flux surface associated with the confinement improvement observed for OW limited plasmas.The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found. Finally, it is shown that the SOL radial transport and the amplitude of the fluctuations increase with plasma current and decrease with line-averaged density for IW limited plasmas.

Calculations of the profiles along the field lines within the divertor and scrape-off layer (SOL) of differences in the plasma ion density, temperature, parallel current, parallel flow velocity, and electrostatic potential, which result from using different poloidal distributions of the particle and heat influxes crossing the separatrix from the core plasma into the SOL, are presented and discussed vis-à-vis experimental observations. The calculations show that the stronger outboard than inboard particle and heat fluxes into the SOL caused by the geometric compression/expansion of flux surfaces predicted by magnetohydrodynamic equilibrium calculations lead to a prediction of higher plasma temperature at the outboard divertor than at the inboard divertor, a result that is consistent with experimental observation and that confirms a previous prediction (made without accounting for drifts) of a possible cause of the observed in-out divertor power asymmetry. The calculations also illustrate the effect of the poloidal distribution of particle and power influx into the SOL on the flow velocity, parallel current, and electrostatic potential distributions in the SOL and divertor.

Models of the electron behavior in the scrape-off layer (SOL) of diverted and limited boundary conditions that occurs tokamak plasmas must retain the abrupt change in boundary conditions that occurs across the separatrix or last closed flux surface as well as the electron reflecting Debye sheath established at the limiter or divertor plates. The balance between ion radial diffusion and streaming to the plates sets the SOL width and the electrons must adjust the Debye sheath at the plates to main tain quasineutrality and ambipolarity in the SOL beyond the last closed flux surface. We consider the long mean-free-path limit where a bounce-averaged kinetic electron model results in a steady-state balance in the SOL between radial diffusive feed from the core and velocity space diffusive loss to the plates due to collisional detrapping. In this double diffusion model a velocity space boundary layer occurs about the trapped-passing boundary where strong velocity space gradients must balance the streaming of the newly de-trapped electrons to the plates. The behavior of the electron distribution function in the velocity provides the information needed to evaluate the Debye-sheath-dependent electron loss rate.

An ERP study of inhibitory control in adults with developmental coordination disorder

Unusually high power loads on the beryllium limiter caused by neutral beam re-ionisation, and much cooler divertor target surfaces were observed during the recent JET tokamak tritium campaign. As both phenomena are driven by scrape-off layer (SOL) physics, the SOL features of 72 tritium H-mode discharges and their deuterium references have been studied. The majority (70) of tritium H-mode discharges had exponentially decaying SOL profiles. The tritium plasmas are observed to have increased separatrix density and collisionality compared to their deuterium references. This is associated with times broader SOL width for both density and temperature profiles. This is consistent with previous observations in highly collisional deuterium H-mode plasma on the ASDEX Upgrade tokamak (Sun et al 2015 Plasma Phys. Control. Fusion 57 125011) and interpreted as high collisionality enhancing cross-field transport across the separatrix and resulting in the broadening of near SOL above a critical value. The other two tritium H-mode discharges had near flat SOL density profiles, similar to the so-called ‘density shoulder formation’ observed in L-mode plasma. The SOL collisionality of these two pulses lies within the range of T pulses without density shoulder formation. This supports the conclusion of previous studies (Vianello et al 2017 Nucl. Fusion 57 116014; Wynn et al 2018 Nucl. Fusion 58 056001) that increased collisionality is not sufficient for the formation of a ‘density shoulder’ and additional factors, likely divertor condition or interaction with neutrals, are required. JET tritium plasma provides evidence of favourable and unfavourable effects of enhanced cross-field SOL transport on machine operation. The larger limiter power loads due to re-ionisation of neutral beam injection observed in the T pulses relative to their D references has been shown to be consistent with the combined effects of the broadening of the SOL profile and larger beam ion Larmor radius. The enhanced cross-field particle transport and the resulting broader SOL width provides more particles to ionize the fast Beam neutrals, causing the unfavourable power load issue on the beryllium limiter. The broader near SOL profiles of the T plasma spreads the heat load over a larger area and, together with the increased separatrix density, results in a favourably cooler divertor target surface.

A model of the scrape-off layer outside the separatrix of a diverted tokamak plasma is used to calculate the current flowing into the divertor plates. The model extends previous work by P.J. Harbour. This current can be due to thermoelectric effects or to an externally applied voltage. It is found that when the target plates are grounded, as in present-day experiments, asymmetries in the power deposition to the target plates can drive current along magnetic field lines in the scrape-off layer. These currents can modify the collisional transport across the separatrix.

Impurity transport in the scrape-off layer (SOL) has been studied in ohmically heated discharges of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy. The vertical profile (from the plasma center to the lower X-point) of carbon emissions of CIII (977 Å: 2s2 1S0-2s2p ) and CIV (1548 Å: 2s 2S-2p 2P) as well as the ratio of CIV to CIII were measured to investigate the edge impurity transport with relation to impurity source locations and sputtering characteristics. The experimental result shows that the impurity profile in the SOL has been clearly changed against different source locations. The emission of CIII and CIV from the mid-plane is stronger than that from the X-point when the impurity source is located at the divertor plate. The profile becomes flat as a result. When the impurity source changes to the dome source, the profile clearly changes to a slightly peaked one, indicating the edge carbon emission at the X-point is stronger than the mid-plane. The change to the limiter source makes the profile further peaked by increasing the carbon emission at the X-point. In the case of the dome impurity source, the intensity of CIII/ne and CIV/ne, normalized to line-averaged electron density, ne, decreases with ne at low ne (ne ⩽ 2.6 × 1019 m−3) and becomes saturated at high ne (ne > 2.6 × 1019 m−3). In contrast, the ratio of CIV to CIII increases with ne at low ne and starts to decrease at high ne. A numerical simulation with 3D edge plasma transport code, EMC3-EIRENE, suggests that a poloidal asymmetry in the impurity flow profile and an enhanced physical sputtering play an important role in the edge impurity distribution, particularly in the screening efficiency of C2+ and C3+ ions in the SOL region of the HL-2A tokamak.

The scaling of edge properties with main plasma parameters in doublet III discharges

It is critical to keep a low level of tungsten impurity concentration in the core plasma of the fusion reactor. Using the OEDGE code, the influence on the tungsten concentration due to the cross-field transport in the scrape-off layer is studied. Based on the design parameters of the China Fusion Engineering Test Reactor, the background plasma with the peak target electron temperature of 15 eV is generated using the onion-skin model. The neon impurity with the assumed concentration of 0.2% inside the separatrix is included for estimating the tungsten sputtering source. In a series of simulations of tungsten transport, the diffusion coefficient D ⊥ is varied from 0.1 to 10 m2 s−1, and the hypothetical inward pinch velocity V IMP is in the range of 0–5 m s−1. The tungsten concentration inside the separatrix increases with decreasing diffusion coefficient and exceeds 10−5 for the diffusion coefficient lower than about 0.2 m2 s−1. When the hypothetical inward pinch is included, the tungsten concentration increases. The relation between the relative increase in tungsten concentration and V IMP/D ⊥ is fitted.